Method for maintaining a desired braking torque

ABSTRACT

A stepped braking system is used as a function of the required braking torque in this method for maintaining a desired braking torque in a vehicle equipped with stepped and infinitely-variable permanent braking systems whereby simultaneous activation of the stepped and infinitely-variable braking systems occurs in such manner that the sum of the participations of the different systems provides the required braking torque at any time.

[0001] The present invention concerns a method for using a wear-free braking system and, in particular, for maintaining a desired braking torque of a motor vehicle, in accordance with the preamble of claim 1.

[0002] In the state of the prior art, nowadays and especially in goods vehicles, permanent braking units (wear-free brake systems) are used to relieve the load on the operating brakes. Permanent braking units can also improve the economy of goods vehicles by allowing higher average speeds (especially on long downhill drives) and by considerably reducing the wear of the brake linings of the operating brakes.

[0003] Engine brake systems are used as permanent brakes which, during a thrust operation, produce a braking torque that depends on the gear engaged. In addition, retarder brakes are known to convert kinetic energy into heat energy, which differ in their manner of energy conversion in that with hydrodynamic retarders, the energy is converted by fluid friction and, with electrodynamic retarders, by means of a magnetic field. Retarders act as virtually wear-free permanent brakes, especially for goods vehicles and locomotives, since they have the advantage of converting the energy to be braked into heat without wear over long periods of time.

[0004] In hydrodynamic retarders, the energy flow of a fluid is utilized for braking, the physical action principle corresponding to that of a hydrodynamic clutch with fixed turbine. According to this, a hydrodynamic retarder comprises a rotor in the fluid flow path and a stator fixedly attached to the retarder housing. When the retarder is actuated, a quantity of oil corresponding to the desired braking performance is introduced into the turbine blade area, which the rotor impels against the stator so that a braking action is exerted on the rotor.

[0005] In electrodynamic retarders, on the other hand, the principle used for braking is that of the action of force in electromagnetic fields. Here, a stator is provided with several energizing coils and attached to the transmission housing. Air-cooled rotors are also provided on the transmission-gear side, which are usually connected to the drive shaft. For braking, the energizing coils are supplied with current. As the rotors pass through the magnetic field, eddy currents are induced which impede the rotary movement of the rotors.

[0006] Depending on their arrangement in the drive train, retarders are divided into primary and secondary retarders, primary retarders being arranged on the engine side and secondary retarders on the transmission side. In the present state of the art, electrodynamic retarders are mainly arranged as secondary retarders. This means that primary retarders operate as a function of the engine speed, while secondary retarders operate as a function of the speed of the vehicle.

[0007] In addition, permanent braking systems are divided into stepped and infinitely-variable systems; stepped systems are the engine brakes and the electrodynamic retarders. In contrast to the infinitely-variable systems, such as hydrodynamic retarders, the braking performance can only be varied in steps.

[0008] Permanent brakes are particularly important in the case when the speed must be kept constant downhill, but this often entails discomfort for the driver.

[0009] At lower drive shaft speeds, hydrodynamic secondary retarders have their system limits, i.e., the braking torque produce is no longer sufficient. In addition, with hydrodynamic secondary retarders and, as a rule at high drive shaft speeds, the power is limited or reduced in order to protect the engine cooling system.

[0010] Accordingly, the purpose of the present invention is to indicate a method for maintaining a desired braking torque with optimum utilization of the wear-free braking systems of a motor vehicle.

[0011] For this, it is proposed to combine the strengths of the available braking systems (e.g., hydrodynamic+electrical secondary retarder, hydrodynamic secondary retarder+engine braking). In this way the weaknesses of the available various respective braking systems can be compensated.

[0012] According to the invention, the method proposed here can be used for any operation mode, including the regulation of a constant speed on a gradient and maintaining a constant braking torque or maintaining a constant delay.

[0013] Thus, for example, the function of maintaining a constant speed on a gradient can be implemented using several mutually independent braking systems, with stepped and infinitely-variable braking systems acting in combination. The function described can, therefore, be implemented over nearly the entire speed range.

[0014] The prerequisite is to know the braking behavior of the different systems. For this purpose, the parameters that characterize the system behaviour of the various braking systems, for example, the performance graphs of the system, are stored in a memory and the actual braking behavior is measured or calculated.

[0015] The performance of the permanent braking system can be improved by using braking systems with different energy balances, thanks to the method according to the invention

[0016] Furthermore, a stepped braking system can also be used in combination with an infinitely-variable braking system to perform the function of maintaining constant speed on a gradient without compromising comfort.

[0017] The combination of braking systems makes it possible to utilize the different strengths of the systems; for example, at the performance limit of the hydrodynamic secondary retarder the primary retarder or an electrical secondary retarder can be used additionally.

[0018] Below, the invention is explained in further detail with reference to the drawing. The drawing shows:

[0019]FIG. 1 is a representation of the function “constant speed on the gradient” with the aid of a combination of different wear-free braking system in accordance with the invention, and

[0020]FIG. 2 is a representation of the function “constant braking torque” and “constant retardation” with the aid of a combination of different wear-free braking systems in accordance with the invention.

[0021] According to the invention, in order to maintain a constant speed on a gradient, from a certain necessary specified braking torque, a stepped braking system (engine brake, electrodynamic secondary retarder) is activated and the difference to the effective necessary specified braking torque is made up with an infinitely-variable braking system (hydrodynamic retarder).

[0022] According to the invention, the activation of the stepped braking system takes place when the required specified braking torque is at least larger than step 1 of the stepped braking system. The residual torque difference must be large enough for the infinitely-variable system not to be immediately deactivated. For example, if the necessary total braking torque is larger than the braking torque that can be obtained by step 2 of the stepped braking system, then the stepped braking system increases its braking torque to step 2 and the infinitely-variable braking system reduces its braking torque by the same amount. The same applies for the other steps in the same way.

[0023] In this, the stepped braking system can be activated in one or more steps.

[0024] The system limits of the “constant speed on a gradient” function are thus displaced in the direction of lower speeds and a larger speed range can, therefore, be covered.

[0025]FIG. 1 illustrates the way in which the method operates with the aid of a braking torque-time diagram.

[0026] As a function of the required braking torque M brake, a stepped braking system is used with simultaneous activation of an infinitely-variable braking system, in such manner that the sum of the participations of these different systems, at any time t, provides the required braking torque. The step of the stepped system to be activated in each case is calculated from the braking torque required, so that the residual torque difference or the participation of the infinitely-variable system is large enough for the infinitely-variable system not to be immediately deactivated. This ensures that the infinitely-variable system contributes to the braking process to increase the driver's comfort.

[0027] To implement the “constant braking torque” or “constant retardation” function, the procedure is as shown in the braking moment-speed diagram of FIG. 2. Here too, the stepped braking system is activated when the necessary specified braking torque is at least larger than the first step of the stepped braking system. The residual torque difference must again be large enough for participation of the infinitely-variable system not to become zero.

[0028] Thanks to the method according to the invention, the characteristic feature of primary retarders to give rise to abrupt braking torque changes when a gear change occurs can be compensated. It is also possible, when changing down in thrust operation to compensate for the lack of braking torque during the gear-shift process by maintaining a constant braking torque or retardation independently of gear-shift processes. 

1. Method for maintaining a desired braking torque in a vehicle equipped with stepped and infinitely-variable permanent braking systems, characterized in that as a function of the necessary braking torque, a stepped braking system is used with simultaneous activation of an infinitely-variable braking system, in such manner that the sum of the participations of the different systems produces the necessary braking torque at any time.
 2. Method according to claim 1, characterized in that the step of the stepped system to be activated in each case is calculated from the required braking torque, in such manner that the residual torque difference or participation of the infinitely-variable system is large enough for the infinitely-variable system not to be deactivated immediately, whereby the infinitely-variable system contributes to the braking process in order to increase the driver's comfort.
 3. Method according to claim 1 or 2, characterized in that characterizing parameters of the system behaviour of the various braking systems are stored in a control unit, and the actual braking behaviour is measured or calculated.
 4. Method according to any of the preceding claims, characterized in that the desired braking torque to be maintained is obtained from the functions “constant speed on a gradient”, “constant braking torque” or “constant retardation”. 